The present invention relates to an image information compression apparatus for compressing medical information such as medical X-ray image data by block orthogonal transformation.
Recently, in order to save memory capacity for recording image data or to communicate data at a high speed, image data compression techniques have been developed. Known examples of data compression methods are the lossless compression method and the "lossy" compression method. In the lossless compression method, when compressed image data is decoded, a complete original image can be obtained. That is, the expansion property is good. On the contrary, in the "lossy" compression method, a complete original image cannot be obtained when compressed image data is decoded, resulting in poor reproducibility. However, the lossy compression method has a higher compression ratio than that of the lossless compression method.
Examples of "lossy" compression methods are orthogonal transformation and the predictive coding. Each of the two methods may use block coding in which the original image is divided into a plurality of blocks and compressed or a coding method in which the original image is compressed without being divided into blocks.
Generally, the orthogonal transformation and coding method with block coding has been widely adopted. More specifically, a block cosine orthogonal transformation and coding method using cosine transformations is used. This block cosine orthogonal transformation and coding method is disclosed in "Transform Coding of Images, R.J. CLARK Academic Press, 1985".
Conventionally, in order to compress image data using the block cosine transformation and coding method, an original image is first divided into a plurality of blocks, and each block is orthogonally transformed by discrete cosine transformation. A bit allocation table is formed in accordance with a transformation coefficient obtained by this orthogonal transformation. Then, the orthogonally-transformed image data is quantized in accordance with data of the bit allocation table.
When medical image data is to be compressed by the conventional data compression apparatus as described above, a bit allocation table is formed for an image, i.e., an original image. In this case, the original image includes a background portion in addition to a human body portion which is the region of interest. Therefore, if the bit allocation table is formed on the basis of image data of the original image, bit allocation for properly compressing image data of the human body portion as the region of interest cannot be performed due to the undesired background portion. As a result, a sufficient compression ratio cannot be obtained for the image data, thereby degrading the image quality.